1. Field of the Invention
The present invention relates to a method on localization message process for supporting the mobility of wireless nodes and a computer readable recording medium for storing the same; and more particularly, to a method on localization message process to quickly measure a location of a mobile node through a sequence of messages and a link control process in a wireless sensor network and a computer readable recording medium storing the same.
2. Description of Related Art
As a wireless communication technology and a micro electronics technology have been advanced, a wireless sensor network has been proposed. The wireless sensor network is a network formed of low price and midget sensors. The wireless sensor network has been used in various application fields such as safety and security, ecology monitor, an intelligent traffic system, the automatic control of manufacturing process, the administration of physical distribution, and disaster relief. Some of sensor devices essentially require location information. These sensor devices may be a sensor device attached to a patient for remotely diagnosing a medical condition of a patient in a hospital, a sensor device attached to a worker in a construction field, or a target node for accomplishing an individual task in a wireless sensor network for managing employees or assets.
A mobile sensor node has unique characteristics, mobility. That is, the mobile sensor node can change its location in an operation environment. Accordingly, it is necessary to measure a location of the mobile sensor node frequently or in real time.
A typical localization method uses an apparatus for measuring location information. A global positioning system (PGS) receiver is one of representative apparatuses for measuring location information. The GPS receiver provides accurate location information. The GPS receiver receives visual information from more than three satellites and calculates a global coordinate of the GPS receiver by measuring a distance based on a time difference of the visual information from more than three satellites. The GPS receiver may measure location information within an allowable error of about 20 m although there is difference according to a receiving environment. The GPS receiver is significantly effective in an outdoor place which is widely open to smoothly receive target satellite signals. However, the GPS receiver is not an effective device to measure location information in an indoor place or for an application service system that requires very high accuracy such as accuracy higher than the allowable error.
Therefore, a wireless sensor network does not use a GPS due to limitation in applicability and low accuracy. On the contrary, the wireless sensor network requires a localization method for finding a location of a mobile node.
The wireless sensor network includes a sink node configured to collect data sensed by each sensor node and transmit the sensed data to a central application server through a transfer network, a cluster head which is a source of cluster configuration, an anchor node configured to provide an origin point of a geometric location in a cluster and to forward a received message to a remote target location, and a mobile node. Except a mobile node, all of sensor nodes in a predetermined area have unique location information. Geometric location information of the sink node, the cluster head, and the anchor node are set manually when the wireless sensor network is established. Or, the location information of the sink node, the cluster head, and the anchor node are automatically or remotely set by a central server. On the contrary, since a mobile node or a target node is attached to a movable object or a person, the mobile node or the target node may have location information changing according to a location thereof.
A location of a mobile node is calculated by measuring distances between each of anchor nodes and a mobile node and estimating a location of the mobile node based on the measured distances. Such a localization of a mobile node may be performed one of a cluster head, a sink node, and an application server according to power efficiency and computation capability of each node. The distance between an anchor node and a mobile node is measured using parameters such as a received signal strength (RSS), a time of arrival (TOA), or a time of flight (TOF), a time difference of arrival (TDOA), and an angle of arrival (ADA). Accordingly, a mobile node measures a distance to each of anchor nodes sequentially. Here, in order to two dimensional (2D) or three dimensional (3D) location information, it is necessary to measure distances from a mobile node to at least three anchor nodes for 2D location information or measure distances from a mobile node to at least four anchor nodes for 3D location information. Such a process also requires a mobile node not to move. Multilateration may be used to measure a distance. Using Multilateration, it is possible to accurately estimate location information of a mobile node.
However, in a predetermined area, a mobile node may move continuously by a person or an animal, a plurality of mobile nodes are present, or a plurality of mobile nodes move at the same time. In these cases, it is difficult to measure a distance from a mobile node to each of anchor nodes because a mobile node travels at a predetermined speed while measuring the distances to the anchor nodes. Therefore, the location of the mobile node may be changed in proportion to a moving speed thereof or because of message retransmission or back off of measurement try, which are generated by collision between measurement messages generated during measuring distances of a plurality of mobile nodes. Such a conventional localization method has disadvantages of difficulties of accurately measuring a distance between nodes in a low speed mobile environment.
Accordingly, there is a demand for developing a method for processing mobile node localization messages in a wireless sensor network for accurately recognizing locations of mobile nodes even when a plurality of mobile nodes move at a predetermined speed and at the same time.